Planetary gear bearings for a transmission typically have a relatively small diameter envelope. Therefore, the diameter of the bearings and number of rollers in the bearings are limited, especially when a cage is used to retain and separate the rollers. A planetary gear bearing must employ a cage to reduce roller friction in high speed applications. The size and number of rollers in a planetary gear bearing determine the load carrying capacity of the bearing. However, known configurations of the cage bars that separate rollers in the cage take-up excessive circumferential space, limiting the size and number of rollers that can be placed in the cage. As a result, the loading bearing capacity of a known planetary gear bearing is similarly limited.
The torque requirements in modern transmissions are steadily increasing. However, attaining sufficient load bearing capacity in a planetary gear bearing is at times not achievable due to the limitations, noted above, on the number and size of rollers that can be placed in a known cage for a planetary gear bearings.
FIG. 19 is a perspective view of flat steel strip 300 in an initial stage for a prior art roller bearing cage. FIG. 20 is a perspective view of strip 300 in FIG. 19 after slots have been punched out to form pockets. FIG. 21 is a perspective view of a prior art roller bearing cage formed from the steel strip shown in FIGS. 19 and 20. FIGS. 19 through 21 are taken from FIGS. 1 through 3, respectively of commonly-owned U.S. Pat. No. 6,330,748. FIGS. 19 through 21 illustrate a known method of fabricating a cage, for example, for a planetary gear bearing. FIG. 19 shows strip 300 of steel, having a thickness 302. In FIG. 20, slots 304 have been punched out of strip 300 to form pockets for roller bearings. Bars 306 are formed between openings 304. Thickness 308 of bars 306 is equal to thickness 302 of strip 300. Dimension 310 of bars 306, between openings 304, is considerably greater than thicknesses 302 and 308.
In FIG. 21, strip 300 from FIG. 20 has been wrapped to form cage 312. Ends E1 and E2 of strip 300 are joined to form cage 312. Rollers (not shown) are placed in slots 304. Radial dimension 314 of bars 306 is equal to thicknesses 302 and 308. Circumferential dimension 316 of bars 306 is considerably greater than dimension 314. The extent of dimension 316 limits the number and size of rollers usable with cage 308. U.S. Pat. Nos. 1,598,025 and 1,894,595 also illustrate bearing cages having bars between spaces with circumferential dimensions considerably greater than radial dimensions.
It is not possible to successfully increase the number of rollers in a bearing cage using known production methods. For example, reducing the circumferential extent of the bars to accommodate more rollers drastically reduces cage robustness, which typically leads to premature failure.